1. Field of the Invention
The present invention relates to a printing apparatus and a printing method that cause a print head having ink-ejecting nozzles to print on a print medium as it scans over the print medium.
2. Description of the Related Art
Inkjet printing apparatus uses a print head having a plurality of ink-ejecting printing elements (or nozzles) and performs a printing operation by repeating a print scan that causes the print head to squirt ink as it moves over the print medium. Among a plurality of nozzles there are some unavoidable variations in an ink ejection volume and in an ejection direction. These variations may result in a density unevenness and stripes showing up in a printed image.
As a method of alleviating such troubles as density unevenness and stripes, multi-pass printing has been known, for example, as disclosed in Japanese Patent Laid-Open No. H05-031922 (1993). Multi-pass printing divides binary image data for a unit area of a print medium into pieces of image data to be printed one in each of a plurality of print scans and then successively prints the divided pieces of image data in a plurality of print scans, with a print medium convey operation interposed between adjoining print scans. With this arrangement, dots printed by one and the same nozzle are not formed in succession in the print scan direction of the print head, which in turn can spread ejection characteristic variations, if any, of individual nozzles over a wide area. As a result, a uniform and smooth image can be obtained.
As described above, the multi-pass printing needs to divide binary image data into pieces for the multiple passes or print scans. Generally, such a dividing operation often uses a mask pattern composed of a matrix of printable pixels (1) each of which permits a dot to be formed therein and unprintable pixels (0) each of which does not permit a dot to be printed therein. In that case, a logical computation is performed between the binary image data to be printed in each unit area of the print medium and the mask pattern, dividing the binary image data for the unit area into pieces of binary image data that are to be printed in the multiple print scans.
The mask pattern is generally arranged so that the printable pixels (1) assigned to a plurality of print scans are in a complementary relationship. That is, those pixels that are determined by binary image data as being printable (1) are each formed with a single dot in one of the multiple print scans. This arrangement is designed to preserve, even after the dividing operation, the image information that has existed before the dividing operation.
In recent years, however, density changes caused by dot position (or registration) misalignment among multiple print scans have come to be spotlighted as a newly recognized issue.
FIG. 3 is a conceptual diagram showing how a print position misalignment occurs when a 4-pass printing is performed. In a 4-pass printing, unit areas on a print medium are each printed with a plurality of dots in each of the four print scans. The plurality of dots printed in each of the multiple print scans can be taken as a dot group formed on a single plane as shown. Then, a print position misalignment between different print scans over the same unit area can be regarded as a relative position misalignment among dot groups printed in different print scans, i.e., as a misalignment between different planes. For example, when a print medium conveyance error occurs in a unit area of interest between a first pass and a second pass, only a plane corresponding to the first pass is placed out of alignment with the second and third plane. Such a print position misalignment can be triggered unexpectedly by a change in a distance between a print medium and a nozzle face of the print head (head-medium distance) or by a change in the distance that a print medium is conveyed.
Such a print position misalignment, when it occurs, causes many of the dots, that are determined by the mask pattern to be printed at different positions in different print scans, to overlap each other. As a result, the dot coverage ratio on the print medium (or area factor) decreases, lowering the density in the unit area. Further, if unit areas with such print position misalignments and those with no such misalignments are intermingled on the same print medium, the above phenomenon is perceived as a density unevenness.